1. Field of the Invention
The invention relates to polymers for electronic and optical applications and the synthesis thereof.
2. Related Technology
Organic semiconductors are attracting increasing attention across a wide range of applications due to their advantageous electronic properties and their processability. One class of opto-electrical devices is that using an organic material for light emission (an organic light-emissive device or “OLED”) or for light absorption for the purpose of power generation or light detection (a photovoltaic device). The basic structure of these devices is a semiconducting organic layer, sandwiched between a cathode for injecting or accepting negative charge carriers (electrons) and an anode for injecting or accepting positive charge carriers (holes) into or from the organic layer. For example, an OLED is typically fabricated on a glass or plastic substrate coated with a transparent first electrode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium. Other layers can be added to the device, for example to improve charge injection from the electrodes to the electroluminescent material. For example, a hole injection layer such as poly(ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS) or polyaniline may be provided between the anode and the electroluminescent material. In a practical device one of the electrodes is transparent, to allow the photons to escape or enter the device.
In the case of an OLED, holes are injected into the highest occupied molecular orbital (HOMO) of the electroluminescent material and electrons are injected into its lowest unoccupied molecular orbital (LUMO). Holes and electrons then combine to generate excitons which undergo radiative decay, the wavelength of emission being at least partially dependant on the HOMO-LUMO bandgap. Organic materials for use as light-emissive materials include polymers such as poly(p-phenylenevinylene) (as disclosed in WO 90/13148), polyfluorenes and polyphenylenes; the class of materials known as small molecule materials such as tris-(8-hydroxyquinoline)aluminium (“Alq3”) as disclosed in U.S. Pat. No. 4,539,507; and the class of materials known as dendrimers as disclosed in WO 99/21935. These materials electroluminesce by radiative decay of singlet excitons (i.e. fluorescence) however spin statistics dictate that up to 75% of excitons are triplet excitons which undergo non-radiative decay, i.e. quantum efficiency may be as low as 25% for fluorescent OLEDs and so these materials or similar materials capable of transporting charge may be used as hosts for dopants comprising heavy metal complexes capable of harvesting triplet excitons for radiative decay (phosphorescence) as disclosed in, for example, Pure Appl. Chem., 1999, 71, 2095, Materials Science & Engineering, R: Reports (2002), R39(5-6), 143-222 and Polymeric Materials Science and Engineering (2000), 83, 202-203.
Polyfluorenes having a repeat unit of formula (A) are disclosed in for example, Adv. Mater. 2000 12(23) 1737-1750:

wherein R″ represents a solubilizing group such as n-octyl.
These polymers have attracted considerable interest as electroluminescent materials because they are solution processable and have good film forming properties. Furthermore, these polymers may be made by Yamamoto or Suzuki polymerization, for which the appropriate monomers are accessed simply by halogenation of fluorene to form a 2,7-dihalofluorene. These polymerization techniques enable polymerization of fluorene monomers with a wide range of aromatic co-monomers and afford a high degree of control over regioregularity of the polymer. Thus, the physical and electronic properties of polyfluorenes may be tailored by appropriate selection of monomers.
Linkage of the fluorene repeat units through the 2- and 7-positions is important for maximization of conjugation through the repeat unit.
A focus in the field of PLEDs has been the development of full color displays for which red, green and blue electroluminescent polymers are required—see for example Synthetic Metals 111-112 (2000), 125-128. To this end, a large body of work has been reported in the development of electroluminescent polymers for each of these three colors with red, green and blue emission as defined by PAL standard 1931 CIE co-ordinates.
A difficulty encountered with blue electroluminescent polymers to date is that their lifetime (i.e. the time taken for brightness to halve from a given starting brightness at fixed current) tends to be shorter than that of corresponding red or green materials. One of the factors that has been proposed as contributing to the more rapid degradation of blue materials is that their LUMO levels, and consequently the energy level of the charged state following injection of an electron into the LUMO, tend to be less deep (i.e. relatively low electron affinity) than those of corresponding red or green materials. It is therefore possible that materials comprising these lower electron affinities are less electrochemically stable and so more prone to degradation.
For simplicity, a full color display will preferably have a common cathode material for all three electroluminescent materials. Thus, the problem of a large energy gap between the LUMO and the workfunction of the cathode for a typical blue electroluminescent material is likely to be exacerbated where a common cathode suitable for red and green materials is employed.
A blue electroluminescent material having a higher electron affinity than polyfluorenes or a material capable of injecting electrons into blue electroluminescent polymers is therefore desirable, however increasing the electron affinity of a wide bandgap material will tend to result in a smaller bandgap thus making the material less suitable as a blue emitter or as an electron transporting material for a blue emitter.
A further drawback of polyfluorenes is that blue electroluminescent polyfluorenes have a tendency to shift over time towards longer wavelengths, i.e. towards a redder colour of emission. This effect is believed to be due to oxidative degradation and aggregation of the polymer.
EP 1318163 discloses a monomer of formula (B), and electroluminescent polymers derived therefrom:

Likewise, JP 2003-206289 discloses a monomer of formula (C) and polymers derived therefrom:

These disclosures teach formation of the above dibenzosilole monomers either via (a) lithiation of the 2- and 7-positions of the corresponding non-halogenated compound followed by halogen exchange, or (b) by the following process:

In case (a), the alkoxy groups serve to direct lithiation at the adjacent 2- and 7-positions. Likewise, in case (b) the alkoxy groups serve to direct bromination in the same way. Although these alkoxy groups are significant in monomer synthesis, they are likely to cause repeat units derived from such monomers to suffer from steric interference with adjacent repeat units resulting in a twist in the polymer backbone and loss of conductivity. Furthermore, the electron donating nature of these alkoxy groups decreases the electron affinity of polymers derived from these monomers.
A further drawback of polymers derived from monomers (B) and (C) is that the phenyl and methyl groups of these monomer do not afford solubility in common organic solvents such as xylene.
It is therefore an object of the invention to provide a wide bandgap polymer having higher electron affinity than a polyfluorene, i.e. a material capable of blue emission and capable of serving as an electron transporting material for other blue and smaller bandgap emissive materials. It is a further object of the invention to provide such a polymer that does not suffer from undesirable steric effects; that does not suffer from a color shift over time; and that is readily soluble in common organic solvents. It is a yet further object of the invention to provide a host material for luminescent dopants, in particular phosphorescent dopants.